Space-based radar is space-borne radar systems that may have any of a variety of purposes. A number of earth-observing radar satellites, such as RADARSAT, have employed synthetic aperture radar to obtain terrain and land-cover information about the Earth.
Military
In the United States, Discoverer II was a proposed military space-based radar program initiated in February 1998 as a joint Air Force, DARPA, and NRO program. The concept was to provide high-range-resolution ground moving target indication, as well as SAR imaging and high-resolution digital mapping. This program was cancelled by Congress in 2007. SBR is a less-ambitious version of Discoverer II. Space-based radar is a proposed constellation of active radar satellites for the United States Department of Defense. The SBR system would allow detection and tracking of aircraft, ocean-going vessels, and potentially land vehicles from space. This information would then be relayed to regional and national command centers, as well as E-10 MC2A airborne command posts.
a wind scatterometer to measure wind speed and direction
After Seasat, SARs, altimeters and scatterometers have been flown on several other space missions. While the SAR, in principle, is similar to its airborne counterparts, the other two are specific to satellite operations. A satellite radar-altimeter is a nadir-looking radar with very high range resolution, which measures the ocean surface topography with an accuracy in the order of few centimeters. Additionally, analysis of the echo amplitude and shape can extract information about the wind speed and wave height, respectively. Some radar-altimeters employ synthetic aperture and/or interferometric techniques: their reduced footprint allows mapping of rougher surfaces like polar ices. A wind scatterometer observes the same portion of the ocean surface from different angles of view as the satellite passes by, measuring the echo amplitude and the corresponding surface reflectivity. Reflectivity being affected by the ocean surface "roughness", which in turn is affected by the wind and also dependent on its direction, this instrument can determine the wind speed and direction. These three types of radar are currently used on several satellites. Scatterometers are of high value for operational meteorology, allowing reconstruction of wind fields on a global scale. Data from radar altimeters are used for the accurate determination of the geoid, monitoring of tides, ocean currents and other large-scale ocean phenomena such as El Niño. SARs applications are many: they range from geology to crop monitoring, from measurement of sea ice to disaster monitoring to vessel traffic surveillance, not to forget the military applications. SAR imaging offer the great advantage, over its optical counterparts, of not being affected by meteorological conditions such as clouds, fog, etc., making it the sensor of choice when continuity of data must be ensured. Additionally, SAR interferometry allows accurate 3-D Reconstruction. Other types of radars have been flown for earth observation missions: precipitation radars such as the Tropical Rainfall Measuring Mission, or cloud radars like the one used on Cloudsat. Like other Earth observation satellites, radar satellites often use sun-synchronous orbits so that diurnal variations of vegetation are ignored, allowing long-term variations to be more accurately measured. Earth-observing radar satellites include:
Most of the radars flown as payload in planetary missions belong to two categories: imaging radars and sounders. Imaging radars: Synthetic aperture radars are the only instruments capable of penetrating heavy cloud cover around planets such as Venus, which was the first target for such missions. Two Soviet spacecraft imaged the planet in 1983 and 1984 using SAR and Radar altimeters. The Magellan probe also imaged Venus in 1990 and 1994. The only other target of an imaging radar mission has been Titan, the largest moon of Saturn, in order to penetrate its opaque atmosphere. The radar of the Cassini probe, which orbited Saturn between 2004 and 2017, provided images of Titan's surface during each fly-by of the moon. The Cassini radar was a multimode system and could operate as Synthetic Aperture Radar, radar altimeter, scatterometer and radiometer. Sounding radars: these are low-frequency ground-penetrating Radars, used to acquire data about the planet sub-surface structure. Their low operating frequency allow them to penetrate hundreds of meters, or even kilometers, below the surface. Synthetic aperture techniques are normally exploited to reduce the ground footprint and, thus, the unwanted echo from other surface objects. The first radar sounder flown was ALSEon boardApollo 17 in 1972. Other sounder instruments flown, are MARSIS on board the European Space Agency's Mars Express probe, and SHARAD on JPL's Mars Reconnaissance Orbiter. Both are currently operational. A radar sounder is also used on the Japanese Moon probe SELENE, launched September 14, 2007. A similar instrument was embarked on the Japanese Martian mission Nozomi.
